The present invention relates to an assembly of refractory plates for use in a three-plate sliding closure unit for controlling the discharge of molten metal from a metallurgical vessel, for example from an intermediate vessel such as a tundish into the mold of a continuous casting plant. More particularly, the present invention relates to such a refractory plate assembly including inlet and outlet stationary refractory plates having aligned respective outlet openings and a movable refractory plate positioned between the two stationary refractory plates and having an outlet opening that is moved into and out of alignment with the outlet openings in the two stationary refractory plates. The outlet stationary refractory plate carries a refractory outlet, for example in the form of a pouring tube or an outlet sleeve. Also, the outlet opening in the outlet stationary refractory plate enables complete draining of molten metal within the outlet opening of the movable plate during movement of the movable plate in a closing direction to a closed position.
Sliding closure units of the three-plate type are employed, as is known, primarily to control the quantity of molten metal to be introduced into a continuous casting mold from an intermediate vessel such as a tundish. During the casting operation, the starting point for control of the discharge of the molten metal is a choke position whereat the movable plate partially blocks or restricts the discharge channel formed of the outlet openings in the three plates. In this manner, control of the cross sectional area of the discharge channel is possible not only in the closing direction of movement of the movable plate, but also in the opening direction of movement of the movable plate. There is one exception to the choke position that normally is at the start of a casting operation when the discharge channel is completely open. In known arrangements the outlet openings in the inlet and outlet stationary plates are positioned in complete alignment so as to be coincident, and also the outlet opening in the movable plate in the open position is coincident with the outlet openings in the stationary plates. During positioning of the movable plate in a choke position, a choking edge of the movable plate is between the outlet openings in the stationary plates and projects into and restricts the cross sectional area of the discharge channel. This causes a sharp redirecting of the discharging stream of molten metal in the discharge channel, such redirected or deflected stream of molten metal extending in the closing direction of movement of the movable plate. The result of such a sharp deflection of the discharge stream is increased flow pressures, particularly below the movable plate at the edge of the outlet opening of the outlet stationary plate that lies in the flow path and in the drainage region, i.e. in the pouring tube or outlet sleeve, therebeneath. In such regions the refractory materials of the plates are subject to exceptionally high thermal and erosive stresses, thus subjecting the plates to undesirable high degrees of accelerated wear. This in turn necessitates the premature replacement of such plates. In addition, deposits from the molten metal, which increasingly restrict the discharge channel, occur primarily at the edge of the outlet opening in the outlet stationary plate that lies in the flow path due to turbulence of flow of the discharge stream.
German No. DE-PS No. 28 36 434 discloses a three-plate sliding closure unit having a discharge nozzle designed, not with respect to the flow conditions within the discharge channel, but with respect to the outlet opening in the outlet stationary plate. Particularly, such outlet opening is designed as an elongated opening extending in the closing direction of the movable plate to ensure that all molten metal within the outlet opening in the movable plate will completely drain therefrom during a closing operation. However, the production of such elongated outlet openings is time consuming and difficult. This particularly is true in refractory plates wherein the elongated outlet opening must be fitted to the inlet cross section of an attachable pouring tube. Also, an outlet stationary plate with an elongated outlet opening, the cross section of which is greater than the cross section of the other outlet openings of the assembly, is significantly weakened, primarily with respect to flexural strength.